This invention generally relates to vacuum interrupters such as are used in electrical power transmission and distribution devices. It more particularly relates to vacuum interrupters with a coil applying an axial magnetic field parallel to an arc current path established between contacts separated during current interruption.
Common vacuum interrupters include the type in which a coil applies an axial magnetic field parallel to an arc current path established between contacts separated during current interruption. The axial magnetic field improves the interruption performance of the vacuum interrupters (i.e., current interruption capability and dielectric strength).
Axial-magnetic-field-type vacuum interrupters are classified into two types. In one type (refer to U.S. Pat. No. 4,115,672 or UK-A-1,529,669), the coil surrounds the outside of a cylindrical envelope of the vacuum interrupter. In the other type (refer to U.S. Pat. No. 3,946,179 or UK-A-1,478,702), the field-generating coil lies behind one contact within a vacuum envelope of the interrupter. The former vacuum interrupter has the advantage of better heat dissipation from the coil and on the other hand, has the disadvantage of a greater outer diameter since its coil surrounds the outside of the cylindrical envelope of the vacuum interrupter. The latter vacuum interrupter is advantageously compact since it has a compact axial-magnetic-field-generating coil within the vacuum envelope. On the other hand, the latter vacuum interrupter has the disadvantage of reduced heat dissipation from the coil since the coil is located within the vacuum envelope. The latter vacuum interrupter also has a disadvantage with regard to durability since the impact of the contacts upon closing the circuit acts on the coil.
Thus, a recent development has been an axial-magnetic-field-type vacuum interrupter having the coil behind one contact and outside of the envelope of the vacuum interrupter, i.e., opposing the outer surface of a circular metal end plate constituting part of the vacuum envelope.
FIG. 1 illustrates this prior vacuum interrupter. The major part of a vacuum envelope 1 comprises an electrically insulating cylinder 2 made of alumina ceramics, a metal cylinder 3 made of Fe-Ni-Co alloy (e.g., Koval) and coaxially hermetically sealed to the insulating cylinder 2, and metal end plates 4 and 5, the stationary-side end plate 4 made of austenitic stainless steel (e.g., SUS 304L) and hermetically sealed to the outer edge of the metal cylinder 3 and the movable-side end plate 5 made of Fe-Ni-Co alloy (e.g., Koval) and hermetically sealed to the outer edge of the insulating cylinder 2. A stationary lead rod 6 extends through the end plate 4 and a movable lead rod 7 extends through the end plate 5. The movable lead rod 7 can move toward and away from the stationary lead rod 6. The inner end of the stationary lead rod 6 has a disc-shaped stationary contact 8 in contact with the inner surface of the end plate 4. The inner end of the movable lead rod 7 has a disc-shaped movable contact 9 opposing the contact 8. The contacts 8 and 9 are made of a composite material including components of Cu, Mo, and Cr, e.g. Cu-25Mo-7Cr represented in terms of weight % and powder-metallurgically produced (referred to as a Cu-25Mo-7Cr composite material). EP-A-0101024 discloses that a powder-metallurgically produced composite material, e.g. Cu-Mo-Cr, will enhance the interruption performance of a vacuum interrupter.
An axial magnetic field generating coil 10 is disposed close to and behind the contact 8 and outside of the vacuum envelope 1. One end of the coil 10 is electrically connected to the stationary lead rod 6 and the other end of the coil 10 is electrically connected to a terminal 11 of a related power circuit. A flange of the cylindrical shield 12 made of SUS 304L is mounted on the inner surface of the metal cylinder 3 coaxially with the insulating cylinder 2. The shield 12 is equipotential to the stationary contact 8. A bellows 13 sealingly connects the movable lead rod 7 to the end plate 5. A bellows shield 14 is mounted on the movable lead rod 7 in front of the inner end of the bellows 13.
The vacuum interrupter of FIG. 1 exhibits advantages in heat dissipation, durability and compactness.
However, the vacuum interrupter of FIG. 1 exhibited interruption performance lower than that of a vacuum interrupter similar to the example of FIG. 1 but having a pair of contacts made of a Cu-0.5Bi alloy. The vacuum interrupter of FIG. 1 was disassembled and inspected in detail. The present inventors discovered traces of erosion where electrical arcing occurred between the movable contact 9 and an inner surface area of the stationary-side end plate 4 mounted on the stationary contact 8. The present inventors concluded from the above inspection that a suitable combination of materials for the stationary contact 8 and the stationary-side end plate 4, and a clearance between the stationary contact 8 and the stationary-side end plate 4 of the vacuum envelope would relieve the arcing erosion and so have a good effect on the interruption performance of the vacuum interrupter shown in FIG. 1.
The above-described combination of materials and the above-described clearance constitute a primary aspect of a problem to be solved in the vacuum interrupter of FIG. 1. However, a polarity effect in the interruption performance of the vacuum interrupter of FIG. 1 constitutes a subordinate aspect of this problem, described right after this.
In the vacuum interrupter of FIG. 1, magnetic lines of force F generated by the coil 10 tend to deflect from the inside portion of the movable contact 9 to the outside portion of the movable contact 9 due to the large distance between the movable contact 9 and the coil 10, so that a polarity effect occurs in the interruption performance of the vacuum interrupter. Specifically, when the potential of the stationary contact 8 is negative this polarity effect reduces interruption performance of the vacuum interrupter to a lower level than when the potential of the stationary contact 8 is positive during alternating current interruption. In detail, when charged particles are emitted from the movable contact 9, they are effectively directed to the stationary contact 8 along the magnetic lines of force F. On the other hand, when charged particles are emitted from the stationary contact 8, some of the charged particles in the inner area near the periphery of the stationary contact 8 will not reach the movable contact 8 but will spread out into the vacuum envelope 1 along the magnetic lines of force F.
In view of this polarity effect, the diameter of the movable contact 39 (refer to FIG. 8) was selected to be greater than that of the stationary contact 8. Tests were carried out on the interruption performance of a vacuum interrupter with this enlarged movable contact 39, but otherwise similar to the example of FIG. 1. In the vacuum interrupter of FIG. 1 in which the diameters of the stationary and movable contacts 8 and 9 are equal, when a positive potential current is applied to the stationary contact 8, the interruption performance is assigned to value 100%, then when a negative potential current is applied to the stationary contact 8, the interruption performance of the same vacuum interrupter is 80%.
On the other hand, corresponding interruption performances of a vacuum interrupter which is similar to the example of FIG. 1 but in which the diameter of the movable contact 39 is 10% greater than that of the stationary contact 8 are 110% and 90%. These results show that the enlarged diameter of the movable contact 39 corrects for the polarity effect on interruption performance since charged particles emitted from the stationary contact 8 are able to reach the movable contact 39 in spite of the curvature of the magnetic lines of force F.
However, new problems were caused when the diameter of the movable contact 39 was enlarged. Specifically, the electric field strength at the periphery of the movable contact 39 markedly increases since the gap between the periphery of the movable contact 39 and the shield 12 equipotential to the stationary contact 8 decreases as the diameter of the movable contact 39 increases. Therefore, the dielectric strength of the vacuum interrupter is reduced and the gap between the periphery of the movable contact 39 and the shield cannot withstand the transient recovery voltage right after current interruption. This problem is quite serious in the case of the interrupter shown in FIG. 1, since the shield 12 surrounding the movable contact 39 is equipotential to the stationary contact 8, and the potential difference between the shield 12 and the contact 9 increases more than in the case of the vacuum interrupter of FIG. 2 in which the shield 23 surrounding disc-shaped stationary and movable contacts 21 and 22 is at an intermediate potential.
In order to resolve this problem, the present inventors invented a vacuum interrupter as shown in FIG. 8 similar to the example of FIG. 1. As shown in FIG. 8, the diameter of the movable contact 39 is 10% greater than the diameter of the stationary contact 8. The movable contact 39 has no axial magnetic field generating coil; rather, the axial magnetic field generating coil 10 is behind the stationary contact 8. An essentially conical moderating shield 40 made of austenitic stainless steel (e.g., SUS 304L) is mounted on and surrounds the movable lead rod 7 right behind the movable contact 39, i.e. on the side of the movable contact 39 remote from the arcing gap between the stationary and movable contacts 8 and 39, in order to moderate the concentration of the electric field near the periphery of the movable contact 39.
Equipotential lines E shown in broken lines illustrate that the shield 40 moderates the concentration of the electric field between the vapor shield 12 and the movable contact 39.
The enlarged base of the moderating shield 40 has an annular curl 40a which curls into the interior of the moderating shield 40. The annular curl 40a of the moderating shield opposes the periphery of the back surface of the movable contact 39. The maximal outer diameter of the annular curl 40a is approximately equal to the diameter of the movable contact 39. When the maximal outer diameter of the annular curl 40a is greater than the diameter of the movable contact 39, the foot of the electrical arc generated in the arcing gap between the stationary and movable contacts 8 and 39 is at the annular curl 40a rather than at the movable contact 39, thus reducing the interruption performance of the vacuum interrupter. On the other hand, when the maximal outer diameter of the annular curl 40a is smaller than the diameter of the movable contact 39, the moderating shield 40 has no effect and the electric field strength in the outer area near the periphery of the movable contact 39 is excessive, thus reducing the interruption performance of the vacuum interrupter. The rounded apex 40c of the moderating shield 40 has an aperture 40b through which the movable lead rod 7 passes. Thus, the moderating shield 40 exhibits a moderately curved outer surface extending from the annular curl 40a to the rounded apex 40c.
However, the vacuum interrupter of FIG. 8 failed to exhibit the expected improvement in interruption performance. The vacuum interrupter of FIG. 8 was then disassembled and inspected in detail. The present inventors discovered traces of erosion due to electric arcing between the movable contact 39 and an inner surface area of the end plate 4 surrounding the stationary contact 8. It was concluded that the enlargement of the diameter of the movable contact 39 could not prevent the foot of the electrical arc from transferring from the stationary contact 8 to the end plate 4 so that ionized vapor emitted from the movable contact 39 will not always reach the stationary contact 8 along the magnetic lines of force F of the coil 10. The present inventors concluded from the above inspection that the suitable combination of materials for the stationary contact 8 and the stationary-side end plate 4 as well as a clearance between a stationary contact and the stationary-side metal end plate of the vacuum envelope would enhance interruption performance of a vacuum interrupter such as is shown in FIG. 8.